Both facebook and twitter (and pretty much everyone in the market of social apps) build their monopolistic walls out of the network effect - we have the users people are interested in so nobody wants to use your service.

Mandating open communication between these services would help to break this monopoly and possibly just naturally evolve into the idealistic goal diaspora was aiming for, small networks of social groups that have news feeds floating between them and the ability to pick and chose from locally hosted and separately hosted feeds.